In an anatomically normal human hearing apparatus, sound waves, which represent acoustical energy, are directed into an ear canal by the outer ear (pinna) and impinge upon a tympanic membrane (eardrum) interposed at the terminus of the ear canal between the ear canal and the middle ear space. The pressure of the sound waves effect tympanic vibrations in the eardrum, which then become manifested as mechanical energy. The mechanical energy in the form of tympanic vibrations is communicated to the inner ear by a sequence of articulating bones located in the middle ear space, to which are generally referred as the ossicular chain. The ossicular chain must be intact if acoustical energy existing at the eardrum is to be conducted as mechanical energy to the inner ear. The ossicular chain includes three primary components: the malleus, the incus, and the stapes. The malleus includes respective manubrium, neck, and head portions. The manubrium of the malleus attaches to the tympanic membrane at a point known as the umbo. The head of the malleus, which is connected to the manubrium by the neck portion, articulates with one end of the incus, which provides a transmission path for the mechanical energy of induced vibrations from the malleus to the stapes. The stapes includes a capitulum portion connected to a footplate portion by means of support crura and is disposed in and against a membrane-covered opening to the inner ear, referred to as the oval window. The incus articulates the capitulum of the stapes to complete the mechanical transmission path.
Normally, tympanic vibrations are mechanically conducted through the malleus, incus, and stapes, to the oval window and to the inner ear (cochlea). These mechanical vibrations generate fluidic motion (transmitted as hydraulic energy) within the cochlea. Pressures generated in the cochlea by fluidic motion are accommodated by a second membrane-covered opening between the inner and middle ear, referred to as the round window. The cochlea translates the fluidic motion into neural impulses corresponding to sound perception as interpreted by the brain. However, various disorders of the tympanic membrane, ossicular chain and/or inner ear can occur to disrupt or impair normal hearing.
Hearing loss, which may be due to many different causes, is generally of two types, conductive and sensorineural. Of these types, conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, damage to the ossicles or the ossicular chain. Conductive hearing loss may often be helped by use of conventional hearing aids, which amplify sound so that acoustic information does reach the cochlea and the hair cells. In other cases, conductive hearing loss can be helped by the use of a middle ear implant, which essentially augments or bypasses the mechanical conduction of the ossicular chain. Some examples of such a middle ear implant can be found in U.S. Pat. Nos. 4,729,366 and 4,850,962 of Schaefer.
In some types of partial middle ear implantable (P-MEI) or total middle ear implantable (T-MEI) hearing aid systems, sounds produce mechanical vibrations within the ear which are converted by an electromechanical input transducer into electrical signals. These electrical signals are in turn amplified and applied to an electromechanical output transducer. The electromechanical output transducer causes an ossicular bone to vibrate in response to the applied amplified electrical signals, thereby improving hearing.
An electromechanical output transducer used for the purpose of causing an ossicular bone to vibrate may be mounted in or near the middle ear. The transducer, also known as a driver, is generally contained in a housing or enclosure, forming an assembly that facilitates the placement of the transducer within or near the middle ear.
In some previous designs, the output transducer assembly is coupled to some part of the middle ear and has its output portion typically coupled to the moving part of the ear, e.g. the stapes or another element in the ossicular chain. The output transducer, which may be piezoelectric, electromagnetic, electrostatic, or another mechanism, is mechanically coupled to the moving portion of the ear to be vibrated.
One method, for measuring the output vibration of the middle ear element to which the output transducer is coupled, is called Laser Doppler Velocimetry (LDV) or Laser Doppler Vibrometry. LDV typically uses a helium-neon laser, or something similar, and can be used to measure the Doppler shift between incident and reflected light from a vibrating surface such as a middle ear element or a middle ear transducer. This Doppler shift measurement can be used to calculate velocity, displacement, or acceleration of a middle ear element or middle ear transducer. LDV equipment can be expensive, and making LDV measurements in the middle ear can be difficult.
An elongate vibratory body, sometimes called a bimorph or bi-element, can be used to drive a bone in the middle ear. Often the bimorph will have two piezoelectric layers or bodies disposed on either side of a central conducting vane. When the top layer is caused to expand by application of an electric field, and the bottom layer is caused to contract by application of an electric field, the elongate body or bimorph will bend. Long after the implantation procedure, methods such as LDV obviously cannot be used due to the device residing within human tissue.
Undesirable changes may occur long after implantation. It is theoretically possible for the vibratory body to be decoupled from the middle ear bone. The growth of scar tissue, tumors, or other growth could impede the movement of the driven middle ear bone or other element and/or the vibrating body. Fluid buildup could also impede the vibrations. For these and other reasons, measuring the vibration of the driven middle ear bone and/or the vibratory body would also be desirable long after the initial surgery.